GROWTH OF PLANTS 127 



form. Urine contains on the average about 2 mg. of auxin per 

 liter. Subsequently, several auxin-like substances have been 

 identified, of which indole-3-acetic acid is the one most widely 

 used in experimental and practical work. It stimulates roots 

 and causes callus and gall formation on stems of plants. 



The physiological action of pure crystalhne auxin is very 

 intense. According to Kogl, 1 mg. of auxin contains about 

 50 million oat units, thus corresponding to an amount of hormone 

 contained in 7 million coleoptiles of corn, for each such tip con- 

 tains about 7 oat units. 



Auxin gets into urine probably from the plants used for food. 

 It is especially abundantly excreted when large amounts of vege- 

 table oils are consumed. But it is probable that bacteria inhabit- 

 ing the digestive tract also participate in its formation; for 

 substances similar to auxin were found among the products of 

 the vital activity of different microorganisms, especially of bac- 

 teria and molds. 



The growth hormone is capable of moving through the plant 

 only in one direction, from the proximal end toward the base, 

 and never moves in the reverse direction from the base to the 

 top. According to Cholodny, this is connected with the dis- 

 tribution of the electric potentials in the stem, the top being 

 charged negatively in relation to the base. That is why the 

 anions of auxin, representing a weak acid, miist move cataphor- 

 etically within the stem in the direction of the positive pole, 

 i.e., to the base. This opinion is confirmed by the fact that a 

 vertically standing coleoptile of oats, placed between electrodes 

 with a high potential difference between them, curve in the 

 direction of the positive pole, this being easily explained in the 

 following manner: The side of the coleoptile turned to this pole 

 charges negatively through an induced charge; and the flow of 

 the hormone is directed to the opposite side, promoting an accel- 

 eration of its growth. 



Auxin produces an accelerating effect only upon the stage of 

 elongation and does not influence the division of cells, which 

 determines the stage of meristematic growth. According to 

 some authors (Haberlandt, Cholodny), plants possess specific 

 hormones that excite cell division. These hormones are elabor- 

 ated in the phloem cells (leptome) of the conductive tissue and 

 have therefore been designated ''leptohormones." Their activ- 



